diff --git a/lib/minify/matthiasmullie-minify/src/CSS.php b/lib/minify/matthiasmullie-minify/src/CSS.php
index 3ba9e551aca..4953738b142 100644
--- a/lib/minify/matthiasmullie-minify/src/CSS.php
+++ b/lib/minify/matthiasmullie-minify/src/CSS.php
@@ -641,7 +641,35 @@ class CSS extends Minify
 
             return $placeholder;
         };
+        // Moodle-specific change MDL-68191 starts.
+        /* This was the old code:
         $this->registerPattern('/\n?\/\*(!|.*?@license|.*?@preserve).*?\*\/\n?/s', $callback);
+        */
+        // This is the new, more accurate and faster regex.
+        $this->registerPattern('/
+            # optional newline
+            \n?
+
+            # start comment
+            \/\*
+
+            # comment content
+            (?:
+                # either starts with an !
+                !
+            |
+                # or, after some number of characters which do not end the comment
+                (?:(?!\*\/).)*?
+
+                # there is either a @license or @preserve tag
+                @(?:license|preserve)
+            )
+
+            # then match to the end of the comment
+            .*?\*\/\n?
+
+            /ixs', $callback);
+        // Moodle-specific change MDL-68191.
 
         $this->registerPattern('/\/\*.*?\*\//s', '');
     }